Scientists expand search for light dark matter

Physicists on the CDMS experiment have devised a better way to search for a particle that, if it exists, would revolutionize our ideas about dark matter. Photo: Reidar Hahn

After seeing possible hints of surprisingly light dark matter earlier this year, scientists on the Cryogenic Dark Matter Search have found a way to improve their search for such particles.

The discovery of low-mass dark-matter particles could tell us that dark matter is more complicated than we originally thought.

Physicists designed CDMS (pictured above) to look for heavy dark-matter particles, the kind predicted by the popular theory of supersymmetry. Supersymmetry posits that every elementary particle we know — the quark, the lepton and so on — has a massive partner particle. One such partner particle could be what we call dark matter.

However, a different theory, currently rising in popularity, predicts the existence of a light dark-matter particle that is just one member in a family of "dark sector" particles.

"We don't know; there could be both heavy and light-mass dark-matter particles," says physicist Dan Bauer, project and operations manager for CDMS and leader of the Fermilab CDMS group. "That's one of the things that has been interesting in the past few years, the realization that dark matter could be every bit as complicated as normal matter."

The CDMS experiment searches for dark-matter particles using a detector filled with germanium and silicon crystals cooled to a very low temperature, about minus 460 degrees Fahrenheit. Atoms in chilled crystals stay very still, making it easier to notice when they are disturbed. If a dark-matter particle knocks against the nucleus of an atom in the CDMS detector, the interaction will release a small amount of heat and charge, which the scientists measure with sensitive electronics.

The lighter the particle administering this kick, the smaller the amount of heat and charge released. That makes low-mass dark-matter particles particularly hard to find.

A modification to the CDMS detector called CDMSlite — "lite" standing for "low-ionization threshold experiment" — combats this problem with the application of a larger voltage across the crystal (a whopping 69 volts instead of the usual 4). This amplifies the signal that low-mass particles release, giving the scientists a much closer look at the energy range where light dark-matter events should appear.

The experiment has now set the strongest limits in the world for detection of a dark-matter particle with a mass below 6 billion electronvolts.

"We are excluding new parameter space that hasn't been probed before," says Pacific Northwest National Laboratory physicist Jeter Hall, who conceived of and helped realize the idea of using higher voltages.

While CDMSlite is not well suited for looking for heavy dark-matter particles — their much-larger signals would saturate the experiment's electronics — CDMS will not give up on its quest for a massive particle. CDMS scientists will operate detectors in different search modes to cover a wide range of dark-matter masses.

In August, Associate Director for Accelerators Stuart Henderson and Chief Operating Officer Jack Anderson presented Fermilab employees with awards for 40 years of service to the laboratory. Fermilab Today congratulates the employees.

In the News

Synopsis: Neutrinoless decays are a no show again

From Physics, Sept. 19, 2013

The neutrino could be its own antiparticle, and this dual role could cause neutrinos to disappear from certain nuclear decays. Tentative evidence for neutrinoless decays appeared in 2004, in conflict with other experimental searches. A new experiment called Gerda has now conclusively refuted this positive result. The nondetection, reported in Physical Review Letters, places the most stringent limits yet on how frequently neutrinoless decays could occur.

Saying that neutrinos and antineutrinos have the same identity (as described by Ettore Majorana in 1932) conflicts with the standard model of particle physics, but it might explain why neutrinos have a much smaller mass than other particles. One way to test for the neutrino's true nature is with double beta decay, in which two neutrons decay simultaneously into a pair of protons, electrons, and antineutrinos. In the Majorana picture, the same reaction could occur with an antineutrino emitted by one neutron being absorbed as a neutrino by a second neutron. In this neutrinoless double beta decay, the electrons would carry away all the emitted energy.

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The Porcelain Press, a monthly notice posted in the laboratory's restrooms, can be accessed from the ESH&Q homepage under the Recent Updates section on the right-hand side of the page.

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Photo of the Day

On the wing

The dragonfly's surroundings show through its transparent wings.
Photo: Barb Kristen, PPD

In the News

CERN preparing clouds for big 2015 LHC switch-on

From TechWeekEurope, Sept. 19, 2013

The IT team building the systems supporting the Large Hadron Collider (LHC) have a lot of work to do in the next two years, as CERN prepares for the next big batch of experiments in 2015.

The LHC will not be in use again until then. Meanwhile CERN boffins analyse the most recent proton collisions, which they believe have proven the existence of Higgs Boson, the so-called "God particle" which explains why things have mass.

Fresh collisions will be launched in two years' time as CERN looks into other phenomena, such as anti-matter. They will create a significant amount of additional data for the IT team to deal with.

When they need to, each of the site's four particle detector hubs – ATLAS, CMS, ALICE and LHCb – take what amounts to 40 million pictures a second, producing 40 petabytes of information. Whilst not all of that information is kept – much of it related to already understood physics rather than interesting new particles – there is still 25GB a second that has to be thrown on standard discs and tapes. That's real Big Data.